Antarctica's Thwaites Glacier. Photo: David Vaughan
There's a fair chance the widest glacier on the planet has passed it's tipping point, in Antarctica, but new data from scientists sheds more light on what this means for the future of the planet.
A study published earlier this month has raised fresh questions about exactly how fast Thwaites Glacier is melting.
One of the study authors described the glacier as now holding on "by its fingernails".
So it's worth unpacking just what this new study, published in Nature Geoscience, is saying, and what it tells us about how the glacier might impact sea-level rise.
The Thwaites Glacier is sometimes called the "doomsday glacier" - a dramatic nickname first used in a Rolling Stone article, though that's not a term favoured by most scientists. More on that point shortly.
The glacier itself contains enough ice to raise sea levels by more than 60 centimetres. But it also acts as a buffer, holding back a large part of the West Antarctic Ice Sheet. If that is released, it will raise sea levels by several metres.
The glacier flows from West Antarctica over the land's surface to the coast and into the Amundsen Sea.
Because of its huge mass, the glacier pushes deep offshore before it begins to lift off the seabed at what is called the grounding line or grounding zone.
We can think of the grounding line as being "like a hinge", connecting the inland part of the glacier to a floating extension known as an ice shelf, according to Matt King, director of the Australian Centre for Excellence in Antarctic Science at the University of Tasmania.
The Thwaites Glacier's grounding line has been steadily melting and retreating in the direction of the ice sheet interior over decades.
What did this new research find?
For the first time, due to recent sea-ice loss - that's the ice on the sea's surface that forms each winter - researchers were able to send down an autonomous underwater vehicle to survey the area of the seafloor that the glacier used to cover.
On the seafloor, more than 600 metres down, they found an isolated region interspersed with long, parallel ridges, mostly around 20cm high, and ranging mostly between 4m and 8m apart.
They worked out these raised ribs, or "back-stepping sedimentary ridges", were left as the ice at the grounding line lifted and settled on the seabed with the tide.
Because the geology and slope of the seabed changes and undulates, there isn't a nice, continual track of these ridges all the way to the present-day grounding line.
So while the researchers are certain they were laid down before we have satellite data - pre-1992 - and they're fairly certain they formed before the 1950s, beyond that they're not sure exactly how old the ridges are.
But by using the distance between them, the researchers were able to work out how fast the grounding line was retreating at the time.
Satellite data between 2011 and 2017 recorded the average rate of retreat of the Thwaites Glacier grounding line was between 0.6-0.8 kilometres per year, or a few metres a day.
But the distance between ridges indicated that for 5.5 months, sometime prior to the 1950s, the grounding line's retreat suddenly sped up to around 2.3km per year.
So what does this tell us?
"It tells us first of all that the glacier seems to have retreated faster than we've previously observed through satellites," Professor King said.
Friction between the part of the glacier in contact with the pinning point slows the flow of the glacier "for hundreds of kilometres upstream", according to earlier research on the Thwaites Glacier.
Rapid melting at the grounding line can allow it to detach from these pinning points, releasing that friction, in turn allowing hundreds of kilometres of glacier to flow more rapidly.
As the grounding line detaches from the pinning point too, it can allow more water to penetrate underneath the glacier, further accelerating melting.
In the short term, this new data could have some very significant implications, according to glaciologist Andrew Mackintosh, head of the School of Earth, Atmosphere and Environment, and chief investigator of Securing Antarctica's Environmental Future at Monash University.
Researchers know that the grounding line is currently stabilised by a small ice shelf weakly held in place by a pinning point.
In a nutshell, it means previous estimates of how much time it will take to lose the Thwaites Glacier may have been too generous.
Predictions on the rate of glacier loss are made using computer modelling. There are myriad parameters plugged into the models, and this latest data will now be incorporated to help refine predictions, according to Professor King.
What does this mean for sea-level rise?
There's no consensus in the scientific community on whether the Thwaites Glacier has already crossed a tipping point where it will be completely lost, regardless of how fast we get emissions down.
"I think there's been some concern over the last decade or so that this glacier system might have already moved into an unstable configuration," Professor King said.
Which is where the problem with the "doomsday glacier" moniker comes in.
It implies that the loss of the West Antarctic Ice Sheet, currently being held back by Thwaites, and which will raise sea levels by many metres, is a foregone conclusion.
Under high emissions scenarios, that is most likely the case.
"We're not going to lose all of West Antarctica and the 5m of sea level that come with it in this century," Professor King said.
"[But] in some of the high-end [emissions] scenarios, there are some fairly dramatic changes going on in West Antarctica by the middle of the century, and by the end of the century that could be well underway."
Right now melting of the Thwaites Glacier is contributing about 4 percent of annual sea-level rise.
Under low-emissions scenarios, we give ourselves the maximum chance of stopping catastrophic ice loss globally, Professor Mackintosh said.
"Thwaites Glacier is only one part of Antarctica. Even if we were to lose Thwaites, retarding the current rate of warming would save much more of Antarctica. The same goes for Greenland and for mountain glaciers.
"Even if Thwaites is lost, we need to make it incredibly clear that a high-versus-low-emissions scenario will result in extremely different outcomes for the world's ice sheets and glaciers."
How much fossil fuel we burn in the coming decades will directly impact how much and how quickly we lose glaciers and ice sheets.
Every low-emissions pathway requires us to stop burning coal, oil and gas very soon, and the International Energy Agency as of last year said there could be no new investment in fossil fuel projects if we're to stand a chance of keeping warming to within 1.5 degrees Celsius.
- ABC